The problem to be solved by the invention is to compare two optical surfaces, spheres or aspheres, which have nominally the same shape. This may done with a Fizeau interferometer system by placing the first surface, called the master calibration surface, in front of the Fizeau reference surface and then making a measurement of the resulting phase difference of the light reflected off the Fizeau reference surface and the master calibration surface. The result is stored, and next, the master calibration surface is replaced by the second surface, called the test surface. Following this, another interferometric measurement is performed on the phase difference of the light reflected off the Fizeau reference surface and the test surface.
Subtracting the second measured phase difference from the first measured phase difference delivers the wanted deviation of the test surface from the master calibration surface. For the correct result to be delivered, several preconditions have to be fulfilled as well as possible: (1) the Fizeau reference surface should not have changed its shape between the two measurements; (2) the wavefronts illuminating the interferometer “cavities” in both the case of the master surface and the test surface should be identical; and (3) the absolute position of the master calibration surface and the test surface in front of the Fizeau reference surface should be identical in both cases. “Absolute position” means an alignment in 6 degrees of freedom, where the rotation around the optical axis is of minor concern for rotationally symmetrical optical surfaces. When the surfaces to be compared are aspherical surfaces tolerances are in the range of a few nm in order to meet the uncertainty targets for the comparison measure.
Consequently, it is a principal object of this invention to assure that the wavefronts and positioning in both cases is substantially identical within the limits required to achieve the precision needed.
Another problem which is solved by the invention is described is as follows. When the surfaces to be compared are aspherical surfaces, it is necessary to provide a Fizeau reference surface which follows the shape of the surface to be compared in such a way that the normal distance of the two surfaces has a constant value, and that the wavefront illuminating the Fizeau reference surface matches the shape of the aspheric Fizeau reference-surface as well as possible.
For the normal distance of the two surfaces to be a constant value, it is necessary that the Fizeau reference surface is also an aspherical surface, very similar to the surfaces to be compared.
To fulfill the second precondition, the known technique is to design so-called null lenses, which are high precision, very expensive, and bulky lens systems, which consist of a larger number of spherical lens elements. In some cases, it is very difficult to design a null system, which meets the requirements for a given aspherical surface, and in all cases, an experienced specialist is needed to design such a null lens. Because very high standards must be met for manufacturing, the cost of material and the cost of labor for building such a null lens are also very high.
Consequently, it is another goal of the invention to provide methods for solving the problem in a very straight forward, deterministic, very cost effective, and elegant way.
Other objects of the invention will appear hereinafter and will be obvious when the following detailed description in read in connection with the drawings.